1. Field of the Invention
The present invention relates to a cellular phone, which has a casing to be opened/closed, and a battery to be removed.
2. Related Art of the Invention
A cellular phone is battery-driven electronic communication equipment, which incorporates a battery in a main body to supply operating power needed by an electronic circuit.
In a conventional cellular phone, it is one of indispensable necessities to store data regarding user's accounting information, failure history, call originating/incoming history including duration of call, parameters (base station information and the like) used for radio communications, and the like. Means for holding such information are classified into means for storage in volatile memories such as a dynamic random access memory (DRAM), and means for storage in nonvolatile memories such as a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), a FLASH ROM, and a ferroelectric random access memory (FeRAM).
The DRAM that is a type of volatile memories is extremely inexpensive, and generally used as main memory means of electronic equipment such as a personal computer. However, since a capacitor inside the DRAM naturally discharges electricity to cause data loss when left unattended, the memory must be continuously refreshed at a given period. Accordingly, when the data are backed up by using the DRAM, a large electric capacity is necessary, and thus use of the DRAM is not suitable for the backup of the cellular phone, which has a limitation on a size of a battery.
Because of use of a flip-flop circuit for a memory element, in the case of the SRAM that is a type of nonvolatile memories, no memory refreshing needs to be carried out. Accordingly, while backup current is about 100 μA for the DRAM, only about 1 μA is enough for the SRAM, making a large electric capacity unnecessary. By using this feature of the SRAM, in a currently sold cellular phone, the SRAM is generally used as a backup memory in emergency in the form of being used in combination with a secondary battery. However, the SRAM has a limitation on a degree of integration because of its feature that the flip-flop circuit is used for the memory element. In addition, because of a higher price of the SRAM compared with the DRAM, a problem of product cost is inherent in the cellular phone.
The EEPROM is an electrically erasable programmable read-only memory. The EEPROM is similar to the SRAM in that data are not erased even if power is turned off, but more inexpensive than the SRAM. On the other hand, data erasure of the EEPROM has drawbacks: (1) voltage higher than 5 V is necessary, (2) a certain amount of bits must be rewritten even in the case of 1 bit rewriting, and (3) erasing/rewriting can be carried out only up to several hundred thousand to several million times. The FLASH ROM is an improvement on the drawbacks of the EEPROM. The FLASH ROM enables memory erasure to be carried out by block units. However, a limitation still remains on the number of erasing/rewriting times.
The FeRAM is a ferroelectric nonvolatile memory. The FeRAM has features: (1) a writing speed is faster compared with the FLASH ROM, and (2) a life is longer compared with the FLASH ROM. However, the FeRAM has a drawback that since data are destroyed when reading is carried out, writing must be carried out again after the reading. Thus far, various improvements have been made to solve this drawback.
By using the features of the above-described various memories, various backup methods have been presented.
A method is available, which protects stored data from being lost by using a nonvolatile memory to provide backing-up by a primary or secondary battery. As a past well-known example regarding a cellular phone for executing protection of such a type, Japanese Patent Application Laid-Open Hei 09 (1997)-055781 discloses a method of holding data by using charging voltage of a capacitor as a power source when a battery power failure occurs, and transferring a content of a buffer memory to an EEPROM by hardware.
In addition, a method that uses not an expensive SRAM but a FLASH ROM is also available. In Japanese Patent Application 2000-304283 that has already been filed, execution of writing in a FLASH ROM by detecting falling-off of a battery cover is described. This application will be disclosed shortly.
However, in the invention described in Japanese Patent Application Laid-Open Hei 09 (1997)-055781, a transfer circuit for the EEPROM must be added, and a large-capacity capacitor for writing a large amount of data must be provided. Thus, miniaturization of a cellular phone becomes difficult.
In addition, in a cellular phone, base station information between the phone and a base station, positional information, and information regarding call must be held at the cellular phone side. Further, by recent popularization of Internet, a browser (software for Internet access) and a mailer (software for electronic mail) have been loaded on the cellular phone. Further, by addition of a color display function or the like, a high-performance cellular phone that needs a large-capacity memory has become general, thereby creating a tendency to increase an amount of data to be backed up more and more. In the invention described in Japanese Patent Application Laid-Open Hei 09 (1997)055781, only data of several tens of bytes can be backed up. Consequently, it is difficult to back up data of kilo byte units or more, e.g., bookmarks of a telephone directory or a browser, and mail addresses. In addition, there is a problem of incapability of processing an expected increase of data to be protected.
With regard to the invention described in Japanese Patent Application Laid-Open 2000-304283, a product that has a battery and a battery cover integrated is now generally available, but the invention cannot be applied to this product.